T cells are divided into two subsets based on their expression of alphabeta or gammadelta T cell antigen receptors. Since gammadelta T cells secrete Thl cytokines, kill infected cells, and expand during a number of different infections in man (up to 50% of all T cells in the peripheral blood), they are likely to play an important role in human immunity to infection. The importance of gammadelta T cells in murine immunity has been established since mice lacking Gammadelta T cells succumb to infections with several bacterial species. Gammadelta T cells are also important in autoimmunity since they regulate murine autoimmune aa T cell responses. We have found that the major subset of human gammadelta T cells uniquely recognize nonpeptide prenyl pyrophosphates, alkylamines, and bisphosphonates as well as specific lymphomas. Prenyl pyrophosphates, such as isopentenyl pyrophosphate, are essential biosynthetic precursors for isoprenoid compounds that are found in both bacteria and man. We have now identified the major bacterial antigen for gammadelta T cells as (E)-4-hydroxy-3-methyl-but-2-enyl pyrophosphate (HMBPP). HMBPP is an intermediate in a bacterial- and protozoal-specific pathway for isopentenyl pyrophosphate (IPP) synthesis and a potent stimulator of gammadelta T cells. Bisphosphonates, which are drugs that are structurally similar to HMBPP, also stimulate Vgamma2Vdelta2 T cells. We now find evidence for the existence of a novel antigen presenting molecule for nonpeptide antigens. We hypothesize that Vgamma2Vdelta2 T cells recognize HMBPP from external pathogens, bisphosphonates, and endogenous IPP that are presented by a novel antigen presenting molecule. The Vgamma2Vdelta2 TCR recognizes a complex of the nonpeptide antigen and the presenting molecule resulting in ?d T cell activation and effector function. In so doing, gammadelta T cells use their TCRs as pattern recognition receptors and contribute to human immunity to infections and tumors and to the control of autoimmune diseases by bridging the innate and adaptive immune systems. Here we propose to further define the molecular basis for the recognition of nonpeptide antigens by gammadelta T cells. In Aim I, we will identify critical amino acids in the Vgamma2Vdelta2 TCR that are required for nonpeptide, tumor, and superantigen recognition. In Aim 2, we will characterize the antigen presenting molecule for nonpeptide antigens. In Aim 3, we will study the pathways that produce and regulate bacterial HMBPP and determine the importance of this pathway in stimulating gammadelta T cells. In Aim 4, we will determine the mechanism and biological function of bisphosphonate recognition. These studies will provide insights into nonpeptide antigen recognition by human gammadelta T cells and should help clarify their role in immunity and autoimmunity.